Method and system for purifying the exhaust gases of a combustion engine

ABSTRACT

A method and system for purifying exhaust gases of a combustion engine or an SCR method for purifying exhaust gases of an internal combustion engine of a vehicle, according to which ammonia gas is exclusively metered in the exhaust gases. The method includes: releasing ammonia gas from at least one solid absorbing matrix where it is stored by sorption; metering the released ammonia gas in the exhaust gases; regenerating the solid absorbing matrix including: generating a refilling ammonia gas by chemically decomposing an ammonia precursor in a biochemical decomposition unit mounted on board the vehicle and storing at least one protein component adapted to decompose the ammonia precursor; directing the refilling ammonia gas to the solid absorbing matrix where it is stored thereon.

The present application relates to a method and a system for purifyingthe exhaust gases of a combustion engine by injecting exclusivelyammonia gas, and more particularly ammonia gas that is released from oneor several solid absorbing matrices where it is stored by sorption.

Legislation on vehicle and truck emissions stipulates, amongst otherthings, a reduction in the release of nitrogen oxides NO_(x) into theatmosphere. One known way to achieve this objective is to use the SCR(Selective Catalytic Reduction) process which enables the reduction ofnitrogen oxides by injection of a reducing agent, generally ammonia,into the exhaust line. This ammonia may be obtained by using differenttechniques. One known technique is based on the use of a solid absorbingmatrix where the ammonia is trapped by sorption. Generally, the solidabsorbing matrix is stored in a container (or tank) (called hereaftermatrix storage container) mounted on the vehicle. According to thisknown technique, the ammonia is released by heating the solid absorbingmatrix, and then the released ammonia is injected into the exhaust line.

This known technique offers high performance since it allows to sendpure NH₃ in the exhaust gases of the vehicle.

However, the main disadvantage of this known technique is the complexityof the refilling procedure (i.e. the solid absorbing matrixregeneration). Indeed, the actual refilling procedure consists ofconnecting the matrix storage container to an external ammonia source,typically a highly pressurized cylinder. In this procedure, the matrixstorage container has to be dismounted from the vehicle for ammonialoading. Moreover, sending a high pressure of ammonia in the solidmatrix results in a huge heat generation due to exothermal sorptionreaction (tens of thousands J/mol NH₃, as a likely order of magnitude),hence the need of a cooling unit as heat sink. High temperature candamage parts of the system and induce long regeneration time.

In view of the above-mentioned disadvantage, there exists a need for animproved method for the regeneration of the solid absorbing matrix.

An object of the present invention is to solve this above-mentionedproblem by proposing an SCR method for purifying the exhaust gases of aninternal combustion engine of a vehicle, according to which ammonia gasis exclusively metered in the exhaust gases, the method comprising astep of releasing ammonia gas from at least one solid absorbing matrixwhere it is stored by sorption and a step of metering the releasedammonia gas in the exhaust gases. According to one aspect of the presentinvention, the method comprises a step of regenerating the solidabsorbing matrix that consists in:

-   -   generating a refilling ammonia gas by chemically decomposing a        ammonia precursor in a biochemical decomposition unit mounted on        board the vehicle and storing at least one protein component        adapted to decompose said ammonia precursor;    -   directing the refilling ammonia gas to the solid absorbing        matrix where it is stored thereon.

Thus, it is proposed an in situ regeneration procedure. In other words,the regeneration of the solid absorbing matrix takes place on board thevehicle. More precisely, the regeneration procedure according to theinvention is based on the decomposition of an ammonia precursor. Suchdecomposition is obtained by using a biochemical decomposition unitmounted on board the vehicle. The biochemical decomposition unitaccording to the invention stores one or several protein component(s)that catalyze a chemical reaction. More precisely, the proteincomponent(s) is(are) adapted to catalyze the hydrolysis (i.e.decomposition) of the ammonia precursor to ammonia. This decompositionresults in the generation of a refilling ammonia gas. The refillingammonia gas is then directed (i.e. transmitted) to the solid absorbingmatrix where it is stored thereon. According to the invention, noexternal ammonia source is used and no disassembly manual operations areneeded for the regeneration of the solid absorbing matrix(ces). Thus,the regeneration procedure according to the invention is simple, fasterand safer.

In a preferred embodiment, a predetermined amount of ammonia precursoris stored on board the vehicle. For example, during vehicle (engine)operation, it is calculated a desired amount of ammonia precursor to beinjected into the biochemical decomposition unit. Such calculation canbe made as a function of information relative to the amount of ammoniagas that has been injected into the exhaust line. In a particularembodiment, such information may derive from data provided by atemperature sensor, a pressure sensor or a flow meter, or anycombination of these sensors. In another particular embodiment, suchinformation may derive from data provided by a device configured tomeasure the concentration of ammonia stored in the solid absorbingmatrix. In another particular embodiment, this information may bederived from an estimation of the consumption of ammonia.

Advantageously, the protein component (stored in the biochemicaldecomposition unit) comprises at least one enzyme. In particular,thermophile-type enzymes are well suited. In a preferred embodiment, thebiochemical decomposition unit can store urease. Urease can be stored inany suitable manner. For example, in a first embodiment urease can beimmobilized in different layers of resin. In a second embodiment ureasecan be fixed on membranes.

The SCR method according to the present invention is aiming at injectingexclusively ammonia gas in the exhaust gases. In other words, no ammoniaprecursor is injected in the exhaust gases.

In a particular embodiment, the ammonia precursor is an aqueous ureasolution.

The terms “urea solution” are understood to mean any, generally aqueous,solution containing urea. The invention gives good results with eutecticwater/urea solutions for which there is a quality standard: for example,according to the standard ISO 22241, in the case of the AdBlue® solution(commercial solution of urea), the urea content is between 31.8% and33.2% (by weight) (i.e. 32.5+/−0.7 wt %) hence an available amount ofammonia between 18.0% and 18.8%. The invention may also be applied tothe urea/ammonium formate mixtures, also in aqueous solution, sold underthe trade name Denoxium™ and of which one of the compositions(Denoxium-30) contains an equivalent amount of ammonia to that of theAdBlue® solution. The latter have the advantage of only freezing from−30° C. onwards (as opposed to −11° C.), but have the disadvantages ofcorrosion problems linked to the possible release of formic acid. Theinvention can also apply to guanidinium formate. The present inventionis particularly advantageous in the context of eutectic water/ureasolutions, which are widely available in gas stations.

According to the invention, no urea solution is injected in the exhaustgases. There is no line (or conduit) for transporting the urea solutionup to the exhaust line and there is no metering device for injecting theurea solution in the exhaust gases. According to the invention, the ureasolution is exclusively transported to the biochemical decompositionunit, where it enters into a chemical reaction with the enzyme so as togenerate the refilling ammonia gas.

In a particular embodiment, the urea solution can be partially stored ina chamber located within the biochemical decomposition unit, before itis chemically decomposed.

In another particular embodiment, the solid absorbing matrix is storedin a first tank and the ammonia precursor is stored in a second tank.Advantageously, the second tank (storing the ammonia precursor) isconnected in a communicating manner to said biochemical decompositionunit, and said biochemical decomposition unit is connected in acommunicating manner to the first tank (storing the solid absorbingmatrix). In a first embodiment, the first tank and the second tank canbe separate storage tanks. In a second embodiment, the first tank andthe second tank can be two separate chambers of a same container.

It should be noted that it exists well known refilling standards andsystems for ammonia precursor, in particular for the AdBlue® solution(commercial solution of urea). The refilling of the storage tank of theammonia precursor is trivial. For example, this can be achieved by usingavailable standard-designed nozzle and/or bottles with dedicatedinterfaces.

In a particular embodiment, the biochemical decomposition unit can belocated below the storage tank of the ammonia precursor. In thisparticular embodiment, a stream (i.e. part) of the ammonia precursor canbe transported towards the biochemical decomposition unit by gravity.

According to the invention, if the ammonia precursor generates water bythermal decomposition, this water is separated from the ammonia,collected and preferably prevented from being stored on the solidabsorbing matrix. Separation of the water from the ammonia, andgenerally from the eventual other thermal decomposition products(generally gases like CO₂), can be made using a liquid-vapour separatorunit. For example, the liquid-vapour separator unit can comprise (or be)a condenser or one or several membranes like disclosed in U.S. Pat. No.4,758,250 for instance, which is a polymeric membrane. The condenser maybe a specific one comprising a specific shaped tube having differentparts at different temperatures (as described in example 2 below); aphase change material, or any other means for cooling and condensing thegases. Alternatively, the condenser may be part of a device alreadyonboard the vehicle, for instance: part of the vehicle air conditioningsystem.

The water collected may be vaporised in the exhaust gases and/or atleast part of it can be stored for instance to be available fordissolving excess ammonia that would pressurize unduly the storage tankof the solid absorbing matrix (see embodiment with pressure relief valvedescribed below).

The ammonia is metered using a gas line which may comprise a non returnvalve close to the metering point.

The method according to the invention uses two separate storage tanks:one for the ammonia precursor and one for the solid absorbing matrixwhich stores ammonia by sorption. As described in patent application WO2006/012903, metal ammine salts (preferably alkaline earth metalchlorides) can be used as solid storage media for ammonia.

Advantageously, the biochemical decomposition unit is equipped with aheater. Such heater can provide the optimum temperature for the desiredactivity of the enzyme or protein. For example, the heater can beconfigured to maintain within the biochemical decomposition unit atemperature range between 30° C. and 60° C.

More generally, the heater is a chamber whose temperature is controlledwithin predetermined ranges; in case the predetermined range falls belowthe temperature of the environment, cooling means will also be madeavailable within the heater. In other words, the heater can either becontrolled so as to rise up the temperature within the chamber orcontrolled so as to cool down the temperature within the chamber.

In a particular embodiment, the heater is configured to work within atleast one predetermined temperature range corresponding to theactivation of the protein component when conversion is needed, andwithin at least another predetermined temperature range corresponding tothe preservation of the protein component, so as to extend its lifetime.

In a particular embodiment of the invention, the heater can compriseresistive heating elements. These resistive heating elements may bemetallic heating filaments (wires), flexible heaters, (that is to sayheaters comprising one or more resistive track(s) affixed to a film orplaced between two films (that is to say two substantially flatsupports, the material and thickness of which are such that they areflexible)) or any other type of resistive elements that have a shape,size and flexibility suitable for being inserted into and/or woundaround the components of the SCR system. PTC (Positive TemperatureCoefficient) elements are more particularly suitable for heating.

In another particular embodiment of the invention, the heater uses thedissipated heat of the engine (for instance, a flow of the liquid enginecooling system) and/or exhaust line (gases) for heating the biochemicaldecomposition unit.

According to the invention, after generating the refilling ammonia gas,it can be compressed by means of a gas pressurisation unit. The functionof this gas pressurisation unit is to compress the refilling ammonia gasto a suitable pressure for absorption by the solid absorbing matrix.

When heating the solid absorbing matrix to release ammonia, it can bethat too high ammonia pressure build up occurs inside the system, due tothermal inertia or to a potential failure of the heating powerregulation. In order to release the pressure above a given set-point,the excess of gaseous ammonia is preferably released by a safety valveand either directly returned to the ammonia precursor tank (preferredembodiment in the case of a solid ammonia precursor), or first dissolvedin an adequate amount of water, for instance coming from the evaporationof the precursor solution the case being, and stored on purpose, and ata composition involving an amount of available ammonia identical to theone of the precursor solution (preferred embodiment in the case of ureaprecursor solutions). In another preferred embodiment, the excess ofammonia released can merely be dissolved in water and the ammoniasolution so obtained can be used later on for thermal ammonia generationand storage on the solid absorbing matrix.

The present invention also concerns a system for applying the SCR methodas described above, said system comprising:

-   -   a first tank mounted on board a vehicle and storing at least one        solid absorbing matrix where ammonia is stored by sorption,    -   means for metering ammonia gas released from the solid absorbing        matrix, in the exhaust gases,    -   means for directing a stream of ammonia precursor solution to a        biochemical decomposition unit mounted on board the vehicle and        storing at least one protein component adapted to decompose said        stream to generate a refilling ammonia gas    -   means for directing the refilling ammonia gas to the solid        absorbing matrix stored in the first tank.

Preferably, the storage tank for the solid absorbing matrix comprises oris connected to a pressure release valve as described above.

The means for directing the stream of ammonia precursor solution to thebiochemical decomposition unit generally comprise a pipe, a valve andeventually a pump, although if the biochemical decomposition unit islocated below the storage tank of the ammonia precursor solution, thestream can merely be generated by gravity.

The means for separating the water from the ammonia may be a condenserand/or a membrane as set forth above.

The means for directing the refilling ammonia gas to the storage tank ofthe solid storage absorbing matrix may be a simple tube (pipe) and saidrefilling ammonia gas may be mixed with other gaseous decompositionproduct(s) like CO₂ for instance.

In one embodiment, the system of the invention also comprises a pressurerelief valve enabling to release pressure above a given set point in thestorage tank of the solid absorbing matrix. Preferably, it alsocomprises means for dissolving the gases so released into a given amountof water and means for returning the so obtained solution to the storagetank of an ammonia precursor solution.

The present invention is illustrated in a non limitative way by theexamples below relying on FIGS. 1 to 5 attached. In these figures,identical or similar devices bear identical reference numbers.

EXAMPLE 1

FIG. 1 is a schematic view of a SCR system according to a particularembodiment of the present invention.

As illustrated in FIG. 1, a liquid solution of an ammonia precursor isstored in a tank [1] and a solid absorbing matrix is stored in a tank[2]. The tank [1] and the tank [2] are connected together in acommunicating manner via a communication line [9]. The communicationline [9] comprises a biochemical decomposition unit [3] and a condenser[4].

The SCR system of the invention is designed to exclusively injectammonia gas (NH₃) in the exhaust gases. Thus, the SCR system of theinvention is simple and efficient, since only pure NH₃ is sent in theexhaust gases (no liquid solution or solid compound is injected in theexhaust gases).

According to one aspect of the invention, the biochemical decompositionunit [3] receives a stream (i.e. part) of the ammonia precursor. Thebiochemical decomposition unit [3] contains an enzyme (for example,urease) that catalyzes the hydrolysis (i.e. decomposition) of theammonia precursor to ammonia. Thus, the biochemical decomposition unit[3] generates the refilling ammonia gas for the regeneration of thesolid absorbing matrix.

FIG. 4 is a schematic view of the tank [2] according to a particularembodiment of the present invention. As illustrated in this example, thetank [2] comprises one cell [20]. The cell [20] comprises two chambers[21] and [22], each containing a solid absorbing matrix. The chamberscan contain similar or distinct type of solid absorbing matrix. Thechambers [21] and [22] are separated by a gas flow channel [23]. Theammonia gas released from the solid absorbing matrices (contained in thechambers [21] and [22]) and (eventually) the refilling ammonia gas(generated by the biochemical decomposition unit) can flow through thechannel [23] towards the condenser [4]. Of course, in another embodimentthe cell [20] can comprise one or more than two chamber(s).

According to another particular embodiment of the present invention, thetank [2] can comprise a plurality of cells connected in series and/or inparallel.

FIG. 5 is a schematic view of the tank [2] according to anotherparticular embodiment of the present invention. As illustrated in thisexample, the tank [2] comprises three cells (A, B, C) containing, forexample, solid materials showing different ammonia sorption properties.The tank [2] is based on a two-stage unit. The first stage comprises thecells A and B, and the second stage comprises the cell C. For example,the cells A and B are filled with magnesium chloride and the cell C isfilled with calcium chloride or barium chloride. One magnesiumchloride-filled cell (for example, cell A) is used for the absorption ofammonia generated by the biochemical decomposition unit [3] while thesecond one (for example, cell B), previously saturated with ammonia(coming from the biochemical decomposition unit [3]), is used to provideammonia gas which is further absorbed in the cell C. When the cell A isammonia saturated and the cell B is empty, the roles of cells A and Bare reversed. This two-stage unit combines the enhanced absorptionproperties of one matrix material (for example, magnesium chloride) forammonia capture, and the advantage of the desorption properties of asecond matrix material (for example, calcium chloride or bariumchloride) to make ammonia readily available for the selective catalyticreduction (i.e. purification of the exhaust gases).

As illustrated in FIG. 1, the tank [2] is connected to the exhaust pipe[5] via an injection line [10] configured to transport exclusivelyammonia gas. According to one aspect of the invention, ammonia gas (NH₃)released from the solid absorbing matrix flows through the injectionline [10] and is metered in the exhaust pipe [5].

According to another aspect of the invention, no liquid solution isinjected in the exhaust pipe [5]. As illustrated in FIG. 1, there is noline (or pipe) for transporting the liquid solution stored in tank [1]up to the exhaust pipe [5]. The liquid solution is exclusivelytransported to the biochemical decomposition unit [3], where it ischemically decomposed to generate the refilling ammonia gas for thesolid absorbing matrix stored in tank [2].

The liquid solution stored in tank [1] is preferably a 32.5% ureasolution commercially available under the brand name Adblue®, but othersoluble ammonia compounds (like ammonium carbamate or guanidiniumformate) are also suitable. A stream (i.e. part) of the solution entersinside the biochemical decomposition unit [3], where water evaporationand urea decomposition occur. The water is further separated in thecondenser [4], and the remaining gaseous flow (i.e. the refillingammonia gas) goes through the tank [2] where ammonia is trapped on thesolid absorbing matrix.

In a particular embodiment, for example when the solid absorbing matrixis saturated with ammonia, a stream (i.e. part) or all of the refillingammonia gas can flow through the injection line [10] and can be meteredin the exhaust pipe [5]. In this particular embodiment, the stream orall of the refilling ammonia gas can flow through the solid absorbingmatrix, i.e. the stream or all of the refilling ammonia gas is nottrapped on the solid absorbing matrix.

As illustrated in FIG. 1, the biochemical decomposition unit [3] isequipped with a heater [31]. The heater [31] can provide inside thebiochemical decomposition unit [3] the optimum temperature for thedesired activity of the enzyme. For example, the heat source of theheater [31] can be derived from a hot part of the vehicle, and ispreferably a section of the exhaust line. Alternatively, the heater canalso be electrical. For example, the temperature range is 30° C.-60° C.

In a particular embodiment, the refilling ammonia gas generated by thebiochemical decomposition unit can be compressed to a suitable pressurebefore absorption by the solid absorbing matrix. To this aim, a gaspressurisation unit (not shown) can be mounted between the biochemicaldecomposition unit [3] and the condenser [4]. In a particularembodiment, the gas pressurisation unit can comprise a pump. In anotherembodiment, the gas pressurisation unit can comprise a piston system. Inyet another embodiment, the gas pressurisation unit can comprise acontrollable valve system.

As regards the content of the tank [2], any material showing ammoniasorption properties is convenient; however, a matrix containing alkalineearth metal chloride is particularly adapted. The excess of carbondioxide is either trapped in the condensed water or released in theexhaust pipe [5], when the pressure inside the tank [2] reaches apre-set level. In vehicle operation, ammonia is desorbed from the solidwhich is stored in the tank [2], and carried to the exhaust line [5],upstream of the SCR catalyst. The condensed water can be furthervaporized in the exhaust line of the vehicle.

In a particular embodiment, an ammonia adsorption loop can also be used,for example in the form of a convection system with a carrier gas usedfor ammonia depletion of the biochemical decomposition unit [3], andtransfer of the ammonia gas to the solid absorbing matrix. Afterabsorption, the ammonia-free carrier gas is available to be enrichedwith ammonia by flowing again through the decomposition unit [3].

EXAMPLE 2

This example, relying on FIG. 2 attached, illustrates the case in whichthe condenser [4] is made of a shaped tube. The tank [1] is filled witha urea solution. The gas flow resulting from the water evaporation andthe urea decomposition goes through the inlet part [4 a] of thecondenser. Water vapors are condensed in part [4 b] of the device,having a temperature lower than part [4 a]. Liquid water is furthercollected in part [4 c], and is removed by opening the valve [6].Ammonia vapor goes to tank [2] through the outlet of the part [4 b] ofthe condenser.

EXAMPLE 3

In this example, the condenser of example 1 is removed and the water/gasseparation is made effective by using a membrane or a series ofmembranes at the outlet of the biochemical decomposition unit [3].

EXAMPLE 4

In this case, tank [1] is filled with a solid state ammonia precursor,for example: urea or ammonium carbamate, in the form of powder, pelletsor flakes. A stream (part) of the solid is drawn to the biochemicaldecomposition unit [3] where ammonia is generated, and further trappedin a solid material in the tank [2]. No separator device (watercondenser, for example) is needed in this example.

EXAMPLE 5

In this example, relying on FIG. 3, a pressure safety function of tank[2] is described in this example. When pressure build up inside the tank[2] is higher than a set value, the pressure valve [8] is open, and theammonia gas flows to the tank [1], through the line [7]. Ammonia isfurther dissolved in the solution which is stored in the tank [1].

In a preferred embodiment, which is not illustrated in FIG. 3, line [7]does not return ammonia directly to tank [1] but instead, it conveys itfirst to a chamber/tank where it is dissolved in an appropriate amountof water so as to reach the right composition (preferably having thesame amount of available ammonia as the solution in tank [1] and theurea solution so obtained is then sent to tank [1].

This can be done for instance by storing a given amount of water in thechamber, by deducing the amount of ammonia released from the pressuredifference since the beginning of the pressure release and by returningthe solution to the tank when the right ammonia concentration isreached.

1-15. (canceled)
 16. A SCR or Selective Catalytic Reduction method forpurifying exhaust gases of an internal combustion engine of a vehicle,according to which ammonia gas is exclusively metered in the exhaustgases, the method comprising: releasing ammonia gas from at least onesolid absorbing matrix where it is stored by sorption; and metering thereleased ammonia gas in the exhaust gases; regenerating the solidabsorbing matrix, including: generating a refilling ammonia gas bychemically decomposing an ammonia precursor in a biochemicaldecomposition unit mounted on board the vehicle and storing at least oneprotein component adapted to decompose the ammonia precursor; directingthe refilling ammonia gas to the solid absorbing matrix where it isstored thereon.
 17. A SCR method according to claim 16, wherein theprotein component comprises at least one enzyme.
 18. A SCR methodaccording to claim 17, wherein the enzyme is urease.
 19. A SCR methodaccording to claim 16, wherein the biochemical decomposition unitincludes a heater.
 20. A SCR method according to claim 16, furthercomprising separating water from the refilling ammonia gas by aliquid-vapour separator unit.
 21. A SCR method according to claim 16,further comprising compressing the refilling ammonia gas by a gaspressurisation unit, the compressing being performed before thedirecting the refilling ammonia gas to the solid absorbing matrix.
 22. ASCR method according to claim 16, wherein a stream of the refillingammonia gas is metered in the exhaust gases.
 23. A SCR method accordingto claim 16, wherein the solid absorbing matrix is stored in a firsttank and the ammonia precursor is stored in a second tank, and whereinthe second tank is connected in a communicating manner to thebiochemical decomposition unit, and the biochemical decomposition unitis connected in a communicating manner to the first tank.
 24. A SCRmethod according to claim 16, wherein the solid absorbing matrixcomprises a plurality of cells containing at least one material adaptedto store ammonia by sorption.
 25. A SCR method according to claim 16,wherein the ammonia precursor is a solid compound.
 26. A SCR methodaccording to claim 16, wherein the ammonia precursor is an aqueous ureasolution.
 27. A SCR method according to claim 16, wherein the first tankin which the solid absorbing matrix is stored is connected to a safetyvalve configured to release excess of gaseous ammonia to release thepressure above a given set-point.
 28. A system for applying an SCRmethod according to claim 16, the system comprising: a first tankmounted on board a vehicle and storing at least one solid absorbingmatrix where ammonia is stored by sorption; means for metering ammoniagas released from the solid absorbing matrix, in exhaust gases; meansfor directing a stream of ammonia precursor solution to a biochemicaldecomposition unit mounted on board the vehicle and storing at least oneprotein component adapted to decompose the stream to generate arefilling ammonia gas; means for directing the refilling ammonia gas tothe solid absorbing matrix stored in the first tank.
 29. A systemaccording to claim 28, wherein the protein component comprises at leastone enzyme.
 30. A system according to claim 29, wherein the enzyme isurease.